A passive optical network (PON) is an optical fiber access technology that enables a cost effective solution for connecting a large number of subscribers. FIG. 1 illustrates an exemplary PON 108 integrated in a communication network context 100. A plurality of subscriber-side Optical Network Unit (ONU) devices 105.1 to 105.n are connected to a central office-side Optical Line Terminal (OLT) device 102 via a passive Optical Distribution Network (ODN) 107, which typically includes only passive components such as optical fibers 103, optical power splitters 104, and optical connections 109. The OLT 102 is typically connected to another network 101, e.g. an Ethernet or the like. The ONUs 105.1 to 105.n, which may be generally referred to herein as ONU 105 and which are connected to the ODN 107 on one side, on the other side are typically connected with respective subscriber networks or subscriber devices 106.1 to 106.n, which act as the end receiver and/or source of the payload data transmitted in the PON.
In a PON, it is common to define the transmission direction from the OLT to the ONUs as the ‘downstream’ direction, and the transmission direction from an ONU to the OLT as the ‘upstream’ direction, and these definitions are used herein. Due to the tree-like topology of a PON, the transmission modes for downstream and upstream are different. For the downstream transmission, the OLT broadcasts optical signal to all the ONUs in continuous mode (CM), and each particular ONU accepts only those downstream frames in the CM stream which headers specify it as the target destination of the frame. However, in the upstream channel, ONUs adopt burst mode (BM) transmission wherein each ONU only transmits in a time slot allocated to it by the OLT, so that signals from different ONUs do not overlap at the OLT. Since the timings of the upstream bursts received by the OLT from different ONUs are not synchronized, a burst synchronization procedure has to be performed by the receiving end of the OLT or any other PON-connected device attempting to extract information contained in the upstream bursts.
One variant of the PON access technology, which is known as Gigabit-capable PON (GPON), supports transmission rates in excess of 1 Gbit/s and is specified in G.984-series of ITU-T Recommendations. In GPON defined by ITU-T G.984, the upstream bursts generated by the ONUs include a 24-bit burst delimiter bit pattern at a pre-determined position within the burst. These burst delimiter bit patterns are defined in messages that the OLT sends to the ONUs at their activation, and are then used by the OLT for the burst synchronization of the received upstream bursts, i.e. to determine the start position of the burst.
FIG. 2 illustrates an exemplary method 200 of establishing upstream burst synchronization in a GPON system, which may be performed by a burst synchronizer function, which may be implemented within an OLT or a test instrument. As shown in FIG. 2, the synchronizer function first receives a bit sequence 201 recovered from the received optical burst signal as known in the art. When the received bit sequence becomes 24 bits or larger, the synchronizer function performs a pattern matching operation 202 starting with first 24-bits received, to detect whether a portion of the bit sequence matches the 24-bit upstream burst delimiter pattern predetermined as a PON system parameter.
If the matching failed, i.e. the delimiter pattern is not detected, the synchronizer function shifts the matching position in the bit sequence by 1-bit, as illustrated by an arrow 202.1. That is, when the current matching operation that was performed starting at bit n of the received bit sequence did not produce a match, the matching operation 202 is then repeated starting at bit n+1 of the received bit sequence, attempting to detect the delimiter pattern again. If the matching succeeds at a particular alignment of the delimiter pattern and the received bit sequence, as illustrated by an arrow 202.2, the synchronizer function determines the next bit following the delimiter pattern to be the beginning position of the data portion of the upstream burst, and performs burst synchronization and data processing 203.
Importantly, the upstream burst delimiter pattern has to be known by the synchronizer function of the burst receiving device in order to perform the matching operation. In GPON systems defined in ITU-T G.984, the delimiter pattern to be used for upstream bursts sent from an ONU to the OLT is set by the OLT as an ONU parameter in the activation process of the ONU, and in general may be an arbitrary bit pattern of 24-bits length. In the most common use case the burst synchronization function is implemented by the OLT when receiving upstream transmission bursts, and is known a priori within the OLT. In the case of a test instrument that is connected within the ODN, e.g. inserted at optical connection points 109, the delimiter pattern of the upstream bursts is generally not known a priori. Accordingly, test instruments that are used for testing upstream transmission parameters heretofore had to obtain the delimiter pattern either by user input or by analyzing an Upstream_Overhead message as defined in ITU-T G.984, which is transmitted by the OLT in downstream direction in the activation process of the ONU. Therefore, prior art upstream transmission test instruments had to either require the user to obtain the delimiter pattern using alternative means and then input it into the tester, or include circuitry for receiving and decoding both the upstream and downstream transmission, which increased their cost and implementation complexity. In cases where only upstream data contains valuable information, such as in upstream transmission testing, it would be desirable to omit downstream receiver facilities in a test instrument.
An object of the present invention is to provide a method and/or device for synchronizing to upstream bursts based on information that is obtainable from the upstream transmission without requiring a downstream transmission decoding.